Female Gaelic surnames differ from male surnames because girls & women could not be a son or grandson! Accordingly, Ó (from Ua meaning grandson of) is replaced with Ní and Mac (meaning son of) is replaced with Nic. These are contractions of the original form of the female surnames "Iníon Uí" and "Iníon Mhic" (modern O and Mac). The naming system worked like this
The forename is placed first and is followed by "Iníon Uí" which in
turn is followed by her great Grandfathers name for example -Iníon Uí
Donaill - meaning daughter of the grandson of Donal. Where Mac is used
the original form was "Iníon Mhic" meaning the daughter of the son of
(name) for example Máire Iníon Mhic Donaill translates as Mary daughter
of the son of Donal
When women married they dropped the ní and
nic and took the title "Bean" (p. ban) meaning wife for example Bean Uí
Dhónaill or Bean Mhic Gearailt.
Obviously surnames could change
with each generation but there is evidence to suggest that surnames
were becoming fixed (like those of today) before the Norman Invasion.
According to Fr. Woulfe, an early authority on Irish surnames, the first
recorded fixed surname is O'Clery (Ó Cleirigh), as noted by the Annals,
which record the death of Tigherneach Ua Cleirigh, lord of Aidhne in
Co. Galway in the year 916. It seems likely that this is the oldest
surname recorded anywhere in Europe.